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Shortcuts for Divisibility by 2,3 and 4

Here are a set of time saving divisibility shortcuts that you can use: Divisibility by 2: How to check a number is divisible by 2? It is very simple, first we should check the units digit of the number, whether it is divisible by 2. A number is divisible by 2, only when its unit digit contains 0, 2, 4, 6, 8. Examples: Is the number 6542646882 divisible by 2? Now, check the units digit of the number, it is 2, so 6542646882 is [...]

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Multiplying by 25 and 125

To multiply by 25 you multiply by 100 (just add two 0’s to the end of the number) then divide by 4, since 100 = 25×4. Note: to divide by 4 your can just divide by 2 twice, since 2×2 = 4. 64×25 = 6400/4 = 3200/2 = 1600. 58×25 = 5800/4 = 2900/2 = 1450. To multiply by 125, you multiply by 1000 then divide by 8 since 8×125 = 1000. Notice that 8 = 2×2×2. So, to divide by 1000 [...]

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Multiplying by 9, or 99, or 999

  ·       Multiplying by 9 is really multiplying by 10-1.         So, 9×9 is just 9x (10-1) which is 9×10-9 which is 90-9 or 81. ·      Let’s try a harder example: 46×9 = 46×10-46 = 460-46 = 414. ·      One more example: 68×9 = 680-68 = 612.        To multiply by 99, you multiply by 100-1.         So, 46×99 = 46x (100-1) = 4600-46 = 4554.         Multiplying by 999 [...]

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The 11 Rule

 To multiply any two digit number by 11:  For this example we will use 34.         ·      S  Separate the two digits in you mind (3__4).         ·    Notice the hole between them!         ·      Add the 6 and the 4 together (3+4=7)         ·       Put the resulting 7 in the hole 374. That’s it! 11 x 34=374 The only thing [...]

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Maths shortcut

Here’s a collection of time-saving math shortcuts, great for back-of-the-envelope estimates Shortcut to multiplication Square Number Tips 1: 2 Digit Number which ends in 5 Step1: Multiply the first digit of number, with the next to its number. Ex: 45 is the number you want to square. 4 x 5 = 20 Step2: Finally add 25 at the end of the result. Answer=2025 Tips 2: Any 2 Digit Number Ex: 47 Step1: Look for the nearest 10 boundary. 3 from 47 to 50 Step2: Since we went up [...]

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